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Lets start with noises then move to replacement.
Engine sounds can be hard to pin down as to their cause. Typically they can be reduced to the following, but and there’s plenty of space for “buts”. This is only a pointer not a method of specific diagnostics.
Bottom end sounds from the crankshaft can be sorted into reciprocating and rotational sounds.
1) Connecting rod bearings make a characteristic double click or thud with each crankshaft revolution. This sound is usually more noticeable at start up or idle and may be very prevalent when slowing from cruising speed to idle, like when you come off a freeway ramp with a stop, because the oil pressure falls low and the clearances move to their maximums. It may also be present at cruise RPMs under light load. It’s a fairly heavy sound. Pin noise is similar but tends to be lighter or more click like in tonal quality. A cracked piston skirt or excessive skirt clearance is also a double click and is mighty hard for the inexperienced ear to tell from pin or rod bearing noises. These noises will sometimes tend to pick up when the throttle is dropped as this reduces compression which allows the offenders more movement over TDC. These sounds can be tested for by running the engine at idle and grounding (shorting) one spark plug at a time. The offending cylinder or cylinders will quiet down when the plug isn’t firing them.
2) Main rod bearing noise is a single heavy thud in time with rotation. Loose dampers or flywheel/flexplates demonstrate a similar cadence but the tonal quality is lighter and higher.
3) Cam gears and chain noise is usually easy to pick out with a stethoscope against the timing cover. Excessive wear can be tested for with a timing light, disconnect the vacuum advance. Then with the timing light firing, run the RPMs up to a couple to 3 thousand and drop the throttle closed. If the timing marks bounce all over the place, it’s a good sign the timing gears and chain need replacing.
Top end noises tend to be more clicking sounds in time to half crankshaft speed.
1) The SBC went through a period of eating cam lobes and lifters. If the wear has reached a point where the hydraulic lifer can no longer compensate it will begin to click. A cylinder that constantly oil fouls its plug is a good sign that the lobe and lifter are gone.
2) Hydraulic lifters get dirty or tired and stop functioning they cause a click.
3) Certainly other problems like bent pushrods and worn rocker balls, loose studs, sticking valves also cause top end clicks. Sometimes the offenders can be found by hand cranking the engine through its timing cycle, as each cylinder is in firing position you can grasp the pushrod in your fingers and try to rotate it. Cylinders with lash problems will allow the pushrod to spin without any effort. Those in good shape will either take some force or not allow any rotation. Another test is to run the engine at idle with the rocker covers off. When pushing on the pushrod side of the rocker, the sound goes away, you found a problem area. This is a messy test that requires you make or buy some partial covers to prevent oil from going everywhere. Expect to be pinched.
Accessory sounds can be found with a stethoscope or disconnecting the drive belts.
Oil pressure can be a problem that results in sounds. This can range from insufficient delivery from the pump due to wear, a failed bypass spring, system leakage resulting from a blown out galley plug or excessive bearing clearance, a failed filter bypass valve combined with a plugged filter or a plugged intake screen or passage. Perhaps insufficient oil drains back from the heads. Any of these could starve the engine for oil resulting in failures that cause noises.
Replacement: How many miles on on your engine? A cam change on a high mileage engine and transmission is not a good idea. It dumps a lot of extra force on tired parts which will soon rebel.
Your cam is bit toward to wild for a low compression engine, you'll quickly find the bottom end power is gone. So a 400 dollar short block is going to cost you for a set of decent pistons. On the good side, at 5000 miles you should be able to put in new pistons and rings with nothing more than a light hone job. Still you're going to start pushing against the $825 the shop wants to do your short block. If you're not doing the piston change labor of tearing the $400 block apart and putting it back together, that will for sure cost you the $825. Then there's the transmission, you cannot put a fresh engine on a tired automatic. It'll just tear the auto up in a month or so and that's another cost. Automatics wear out at about the same rate as an engine, so as the engine's power falls off with age so does the transmissions ability to transfer and manage that power. So you might as well plan on doing the tranny along with the bottom end.
Bogie
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